1. Field of the Invention
The present invention relates to a projector which projects an image.
2. Description of the Prior Art
Conventionally, a projector is arranged opposite to a screen, and focusing is performed manually or automatically. Referring to FIG. 1, there is schematically shown the arrangement of a conventional projector. Reference numeral 1 represents a projector. Reference numeral 2 represents a projector lens. Reference numeral 3 represents image information such as film. Reference numeral 4 represents a back light source. Reference numeral 5 represents a screen. To view an image, focusing of the projector lens 2 is manually or automatically performed according to the distance between the screen 5 and the projector 1.
Subsequently, a conventional automatic focusing method will be described. FIGS. 2A and 2B show the principle of the method. This automatic focusing method is according to a so-called TCL phase detecting method. Reference numeral 6 represents a light receiving device such as a line sensor. Reference numeral 7 represents a focus detecting lens provided separately from the projector lens 2. Reference numeral 5 represents a screen. A.sub.1 to A.sub.3 represent A line chips of the light receiving device 6. B.sub.1 to B.sub.3 represent B line chips of the light receiving device 6. They are paired such as A.sub.1 and B.sub.1, and A.sub.2 and B.sub.2. Light beams having passed at the left and right sides of the lens 7 are incident on an A line chip and a B line chip, respectively.
In in-focus condition as shown in FIG. 2A, the light beams having passed at the left and right sides of the lens 7 are formed into images on the central pair of the chips A.sub.2, B.sub.2, and the output is as shown at S1. In out-of-focus condition as shown in FIG. 2B, the light beams having passed at the left and right sides of the lens 7 are formed into images on chips such as A.sub.3 and B.sub.1 located away from each other, and the output is as shown at S2. Therefore, in-focus condition is automatically obtained by adjusting the lens 7 so that the output of the light receiving device 6 is as shown at S1.
According to the conventional method, however, it is necessary for the projector 1 to be arranged opposite to the screen 5. In addition, at least two independent optical systems (a projector optical system and a focus detecting optical system) are necessary to perform automatic focusing, so that the price of the apparatus increases.